Reactor control: what is the primary function of control rods inserted into a nuclear reactor core?

Introduction / Context:
Maintaining reactor criticality and power levels requires precise control of the neutron population. Control rods are essential safety and control devices in thermal and many fast reactors. They enable operators and automated systems to adjust reactivity quickly and reliably.

Given Data / Assumptions:

• Control rods are typically made from materials with very high neutron-absorption cross-sections (e.g., boron carbide, silver-indium-cadmium alloys, cadmium).
• Reactivity adjustment is achieved by varying insertion depth.

Concept / Approach:
By absorbing neutrons, control rods reduce the number of neutrons available to cause fission events. Inserting rods increases overall absorption, decreasing k_eff; withdrawing rods has the opposite effect. Temperature and pressure are affected secondarily via power changes, but rods do not directly regulate them.

Step-by-Step Solution:

Verification / Alternative check:
Start-up, shutdown, and scram procedures rely on rapid rod insertion to drive the reactor subcritical, demonstrating their neutron-absorption role.

Why Other Options Are Wrong:

• Temperature/pressure are plant parameters influenced indirectly; they are not directly controlled by rods.
• Fuel consumption schedules are operational logistics, not a rod function.
• Converting thermal to fast neutrons is not a rod function; moderators and spectra define neutron energies.

Common Pitfalls:
Confusing coolant-system controls (pressurizers, valves) with neutronic controls; assuming rods directly regulate thermodynamic variables.

Final Answer:
Absorption of neutrons